As far as I can tell, if you do have Inventory Tweaks installed, that's the issue.

I recreated the crash 4 times:1&2: Setting the right click button to either "Up" or "Down" and3&4: Setting the left click button to either "Up" or "Down".

No other button changes seemed to make a difference.

EDIT: I tried changing the InvTweaks.cfg file to modify the keyboard shortcut combos for moving stuff to upper/lower inventory fields, but it had no effect. The shortcut list in the game still defaulted to saying W/S were responsible for it, even though I had changed the shortcuts to LEFT and RIGHT, which should have changed the list to say A/D were responsible.

It seems the W/S keys are hard-coded in the mod to be used for those combos. I submitted a bug report on the Inventory Tweaks github about the issue. There's no real reason to allow changing the shortcut configuration if it doesn't affect anything.

EDIT #2: As far as changing back your key config, if you're using Magic Loader, enter the setup for your mod profile, turn off Inventory Tweaks and then click "Test". Once at the title screen, modify your keys back to their original values and exit. You can then re-enable Inventory Tweaks and all is well.

If you don't have that option, then you can do the following:

Navigate to your .minecraft folder

Open "options.txt"

Find the following lines:key_key.attack:-100key_key.use:-99key_key.forward:17

Your values will likely be different. Change them to the values shown above and save the file.

That's champion. I was going to report it if it could be recreated but you've beat me to it, and yes, I use InvTweaks.

Thanks for your effort on this! Good spot.

I've found that using the right mouse as forward and left as attack means you can dig in a straight line with ease and still have a hand free to pick your nose or whatever takes your fancy during those long, dull digs.

This is a couple weeks old, but I thought I'd mention that I received an email from the Inventory Tweaks Github owner alerting me that this bug was fixed in the 1.2.4 version of the mod, which is now available!